1. Field of the Invention
This invention relates to a process and an apparatus for manufacturing a micro-structured optical fibre.
2. Description of the Related Art
Optical fibres are used for transmitting light from one place to another. Normally, optical fibres are made of more than one material. A first material is used to form a central light-carrying part of the fibre known as the core, while a second material surrounds the first material and forms a part of the fibre known as the cladding. Light can become trapped within the core by total internal reflection at the core/cladding interface.
These conventional fibres are typically produced by well-known vapour deposition techniques, such as MCVD (Modified Chemical Vapor Deposition), OVD (Outside Vapor Deposition) and VAD (Vapor-phase Axial Deposition).
A more recent type of optical fibre waveguide, having a significantly different structure from that of conventional optical fibres, is the micro-structured fibre (also known as “photonic crystal fibre” or “holey fiber”). A micro-structured optical fibre is a fibre made of a same homogeneous material (typically silica), having inside a micro-structure (i.e. a structure on the scale of the optical wavelength) defined by micro-structural elements extending longitudinally along the fibre and having a predetermined distribution. As a micro-structural element it is possible to identify a micro-hole or a filiform element of a different material than the bulk.
The most common type of micro-structured optical fibre has a cladding region showing a plurality of equally-spaced tiny holes, surrounding a homogeneous and uniform central (core) region. A fibre of this type is described, for example, in international patent application WO 99/00685. In a different embodiment, the central region of the fibre may have a central hole, as described, for example, in international patent application WO 00/60388
These two types of fibres convey light in the core according to different optical phenomena.
In the absence of a central hole, propagation of light in the cladding region is forbidden due to the presence of a lowering of the average refractive index with respect to the core region. Such a structure forms a low-loss all-silica optical waveguide that, by appropriately selecting the values of its characteristic parameters, remains monomode for all wavelengths within the transmission window of the silica. The waveguiding mechanism in that case is closely related to that in conventional optical fibres and is a form of total internal reflection between two materials (air and silica) having different refractive indexes.
In the presence of a central hole, propagation in the cladding region is forbidden due the presence of a “photonic band-gap”. The “photonic band-gap” phenomenon, which is analogous to the “electronic band-gap” known in solid-state physics, avoids light of certain frequencies to propagate in the zone occupied by the array of holes, this light being therefore confined in the core region. Propagation of light in fibres showing a photonic band gap is described, for example, in J. C. Knight, J. Broeng, T. A. Birks and P. St. J. Russell, “Photonic Band Gap Guidance in Optical Fibres”, Science 282 1476 (1998)).
Optical characteristics of the above-described micro-structured fibres depend on the number of holes, the holes diameter, the reciprocal distance between adjacent holes and the hole geometrical pattern. Since each of these parameters can broadly vary, fibres of very different characteristics can be designed.
Micro-structured optical fibres are typically manufactured by the so-called “stack-and-draw” method, wherein an array of silica rods and/or tubes are stacked in a close-packed arrangement to form a preform, that can be drawn into a fibre using a conventional tower setup.
In U.S. Pat. No. 5,802,236A, for example, a core element (e.g., a silica rod) and a multiplicity of capillary tubes (e.g., silica tubes) are provided, and the capillary tubes are arranged as a bundle, with the core element typically in the center of the bundle. The bundle is held together by one or more overclad tubes that are collapsed onto the bundle so as to preserve the close-packed arrangement. The fibre is then drawn from the resulting preform, by feeding the preform into the hot region of a conventional draw furnace.
The Applicant has noted that the stack-and-draw manufacturing method has several drawbacks.
The awkwardness of assembling hundreds of very thin tubes, as well as the possible presence of interstitial cavities when stacking and drawing such tubes, may affect dramatically the fibre attenuation by introducing impurities, undesired interfaces and inducing a reshaping or deformation of the starting holes due to the transport of mass from the tube toward the interstitial holes. Other problems of the stack-and-draw method may be represented by the low purity of the tube materials and by the difficulties in producing tubes of the required dimensions and in obtaining the required pattern of holes. Moreover, the relatively low productivity and high cost make this method not particularly suitable for industrial production.
A different technique, that at least partially overcomes the above problems, makes use of the sol-gel process for producing the glass preform. This technique is described for example in EP1172339A1 in the name of Lucent Technologies Inc., relating to a method of making microstructured optical fiber. This method comprises providing a mold with a multiplicity of elongate elements extending into the mold and being maintained in a predetermined spatial arrangement with respect to the mold, introducing a silica-containing sol into the mold and causing or permitting it to gel. After a gel body has been formed, the elongate elements are removed from the gel body and the gel body is removed from the mold. Alternatively, the elongate elements may be extracted after separation of the gel body from the mould. The gel body is then dried, sintered and purified, and the microstructured fiber is drawn from the sintered body. Removal of the elongate members may be accomplished mechanically, by pulling them up from the mould individually or in small groups. Alternatively, the elongate elements may consist of a polymer and they can be removed by chemical or thermal action, e.g. by exposure of the assembly to an appropriate solvent or by pyrolysis, respectively.
The techniques for removing the elongate elements described EP1172339A1 show some drawbacks.
The Applicant has verified that removal by thermal action is impractical, since the heat generated by combustion of the elongate members can cause the crack of the gel body.
As concerns chemical removal, EP1172339A1 do not specify what material of the rods and what solvent can actually be used for this purpose. The Applicant is of the opinion that such a process would in any case affect the integrity of the gel body.
With reference to the mechanical extraction proposed in as described in EP1172339A1, the Applicant has found that there is again the risk of cracking of the gel preform. In particular, the Applicant has experimentally found two causes of gel cracking during the extraction of the elements.
The first cause is adhesion of the gel to the surface of the elements. The Applicant has observed that such adhesion may be broken by imparting a slight torsion to the elements before their extraction from the gel. However, the torsion of the elements causes interfacial stresses and micro-cracks in the fragile gel surface.
The second cause is related to the fact that the diameter of the longitudinal elements may not be constant as a function of axial position. This is especially true for small diameter elements, such as elements with diameter lower than 2 mm. Therefore, a longitudinal portion of an element may have a diameter slightly greater than the hole of the gel in an adjacent longitudinal position along the direction of extraction. During extraction, this longitudinal portion of the element will then be forced into surface contact with the smaller diameter gel hole, resulting in stress and cracking of the gel surface.